Investigation of the Electrocatalytic Reduction of Peroxydisulfate Using Scanning Electrochemical Microscopy.

The elementary steps of the electrocatalytic reduction of S 2 O 8 2- using the Ru(NH 3 ) 6 3+/2+ redox couple were investigated using scanning electrochemical microscopy (SECM) and steady-state voltammetry (SSV). SECM investigations were carried out in a 0.1 M KCl solution using a 3.5 μm radius carbon ultramicroelectrode (UME) as the SECM tip and a 25 μm radius platinum UME as the substrate electrode. Approach curves were recorded in the positive feedback mode of SECM by reducing Ru(NH 3 ) 6 3+ at the tip electrode and oxidizing Ru(NH 3 ) 6 2+ at the substrate electrode, as a function of the tip-substrate separation and S 2 O 8 2- concentration. The one-electron reaction between electrogenerated Ru(NH 3 ) 6 2+ and S 2 O 8 2- yields the unstable S 2 O 8 3•- , which rapidly dissociates to produce highly oxidizing SO 4 •- . Because SO 4 •- is such a strongly oxidizing species, it can be further reduced at both the tip and the substrate, or it can react with Ru(NH 3 ) 6 2+ to regenerate Ru(NH 3 ) 6 3+ . SECM approach curves display a complex dependence on the tip-substrate distance, d , due to redox mediation reactions at both the tip and the substrate. Finite element method (FEM) simulations of both SECM approach curves and SSV confirm a previously proposed mechanism for the mediated reduction of S 2 O 8 2- using the Ru(NH 3 ) 6 3+/2+ redox couple. Our results provide a lower limit for dissociation rate constant of S 2 O 8 3•- (∼1 × 10 6 s -1 ), as well as the rate constants for electron transfer between SO 4 •- and Ru(NH 3 ) 6 2+ (∼1 × 10 9 M -1 s -1 ) and between S 2 O 8 2- and Ru(NH 3 ) 6 2+ (∼7 × 10 5 M -1 s -1 ).